I could think of a few reasons why not. Learning curve being a big one. For startups it's the difference in finding developers. Preference could be another, whether you think imperative code to be more readable, or maybe you enjoy classes or other JS-targeting languages.

I've looked into Om/ClojureScript now a few times and the learning curve is steep and keeps putting me off, but also I fear that if I start a project with it I'll alienate potential future devs and have difficulty gaining traction. Plus, with ES6/CoffeeScript/TypeScript and libraries like this it's more possible than ever to build a beautiful environment to code in that doesn't require such a big jump.

But hey, at least this stuff brings more exposure to ClojureScript. That's definitely a good sign.